Lady parts and baby parts what is a fetus?

A common-sense view of mammalian pregnancy treats the fetus as (a) an organism and (b) co-extensive with the approximately baby-shaped entity developing in the uterus. In this paper, I draw on metabolic accounts of the organism to show that (a) and (b) cannot both be correct: either the fetus is not an organism, or it is considerably more extensive than we tend to think, overlapping considerably—perhaps completely—with its mother. Although other accounts of organisms may have different consequences, I argue that they are all likely to put pressure on the idea that the fetus is a roughly baby-shaped organism. I show that this has consequences for understanding the metaphysics of birth, our nature and persistence, and the ethical dynamics of pregnancy.